Systems have been known for many years which normally use a landline (via telephone exchanges) for direct connection between each client and the central station. However, these are basically a direct current system. Systems currently available may be relatively easily defeated. Therefore an intruder could bypass the correct operation of such systems and burgle premises without being detected. In other words "line security" is poor.
A further disadvantage of present systems is susceptibility to noise signals induced by interference, or to voltage loss over distance, or to accidental line reversal at a telephone exchange, or to earth path leakage, or to line imbalance.
So-called "high security" types may use pulses transmitted along the line, but in order to achieve any effective improvement in line security, either high speed data transmissions must be used (requiring very expensive high frequency lines) or alternatively a low data transmission rate is used and the time taken to acknowledge a change in status is long (reducing the security effectiveness).
In order to reduce the expense of line rental, systems have been developed to "concentrate" or "bunch" several lines together at a remote location. All clients in that area are thereby connected back to the central station via a single line from the said remote location, rather than an individual line each for the whole distance. Methods used include the "bunching block" which simply permits 19 lines to be wired in series and returned via the 20th line to the central station. However, if there is an attack on any one of the 20 lines, it is not possible to determine at the central station, which of the lines were attacked. In the event of an emergency, patrolmen would need to check 19 clients to find the correct one, losing much valuable time. Alternative systems use a "concentrator" which is a multipole switch such as a uniselector. Up to 100 clients near a remote location are sequentially switched and the status relayed to the central station via a single line. This method is slow and relies on mechanical switching. Line security is again poor.
Apart from questions of line security, line rental cost, speed and interference, the major problem for operators of central stations is the workload on operative personnel. At peak periods of activity, many people are required to attend to the equipment is there are several hundred clients. However, stations monitoring several thousand clients are common. An instance of this problem is, where there are several thousand clients with burglar alarms which they all "seal" (activate) upon closing their premises at around 5.00 pm. Because of this, attempts have been made to use the speed of computers to process this large amount of data with minimal delay.
Computerised systems presently available have simply scanned the lines which terminate at the central station. No system has yet been produced which achieves all the features of this invention, namely:
(1) The ability to transmit to the central station, several different statuses for each client. PA1 (2) The achievement of very high line security without need for high frequency lines, yet permitting rapid update of information. PA1 (3) At a remote location, the combination of several branch lines into one trunk line, which couples to the central station. PA1 (4) Freedom from interference, d.c. paths, line reversal or voltage loss problems. PA1 (5) Computer/microprocessor controlled scanning and multiplexing at a rapid rate. PA1 (6) Different states may have different meanings for each client, under program control. PA1 (7) The high reliability, miniaturisation, speed and low cost of solid state circuitry is used throughout. PA1 (8) Local backup system in the event of any system failure or attack. PA1 (9) Compatability for landline, radio channel or other carrier. PA1 (10) Ability to control from the central station, several functions at the client's premises. PA1 (11) Ability to operate independently of computer breakdown or service. PA1 (12) Ability to identify the person operating the client's control panel.
In this specification reference to client and premises may indicate several clients in one building or a client in one each of several remote premises or several distinct areas under the responsibility of one client.
According to the invention an automatic centralised monitoring system capable of monitoring various functions in a plurality of separate premises including monitoring means for installation in said premises coupled to a line driver for transmitting an interogation signal to said monitoring means and a computer controlling said line driver said line driver being under the control of said computer means for initiating interrogation signals to said monitoring means and including means for interpreting reply signals received therefrom whereby a change of status such as a mal-function at any of said separate premises is detectable.
Conveniently the line driver is connected to a plurality of premises through a selector mechanism whereby a serial scanning operation can be performed on all premises connected to said driver.
In addition a grouping means accumulates the signals received from a plurality of line drivers whereby the computer can serially scan each line driver in sequence.
A portable automatic scanner may be provided to scan the signals from a group of premises controlled by a given line driver in the event of a breakdown.